1. Field of the Invention
This invention relates to laser writing, and more particularly to a method and apparatus for laser writing using a diode pumped, mode-locked laser and/or laser/amplifier system.
2. Description of Related Art
The production of an integrated circuit normally begins with a photomask, which is a photographic negative of a layer of the circuit. The photomask for the first layer of the integrated circuit is projected onto a wafer of silicon which is coated with a photosensitive material. The latent image of the circuit pattern for the first layer is then developed, and the silicon uncovered in this process is appropriately treated to change its electrical characteristics. The steps are repeated for each circuit layer using an appropriate photomask.
Two masking techniques used are contact printing, in which the mask is in contact or in extremely close proximity to the photoresist layer, and projection printing, in which the mask is imaged onto the photoresist. Projection printing offers the advantage that the mask is out of contact with the photoresist, thereby avoiding the hazard of accidental abrasion of the photoresist coating or the mask. A disadvantage is that, generally speaking, increases in the resolution of the image lens are accompanied by reductions of the image field; that is, reductions in the mask area that can be imaged onto the wafer.
In the fabrication of semiconductor devices by photolithographic techniques, a semiconductor wafer is coated with a photoresist, and exposed to actinic light projected through a mask. Development and etching of the selectively exposed photoresist defines a pattern on the wafer surface which may be used for establishing diffusion areas, conductor patterns, and the like. Modern integrated circuit fabrication requires several printing steps to be performed successively, with each mask exposure being in precisely controlled registration with previously formed patterns.
Contact printing was later supplanted by one-to-one projection printing of the circuit onto the photoresist material. Reticles have also been used. These reticles are photomasks of one layer of an integrated circuit pattern enlarged ten times and produced on a glass plate.
Photomasks are produced by photographic reduction of computer generated artwork with the use of a raster-scanned beam.